Re: [PATCH v3 2/5] kselftest: vm: Fix mdwe's mmap_FIXED test case

[Florent Revest]

On Sat, Aug 26, 2023 at 12:28 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Tue, Jul 04, 2023 at 05:36:26PM +0200, Florent Revest wrote:
> > I checked with the original author, the mmap_FIXED test case wasn't
> > properly tested and fails. Currently, it maps two consecutive (non
> > overlapping) pages and expects the second mapping to be denied by MDWE
> > but these two pages have nothing to do with each other so MDWE is
> > actually out of the picture here.
> >
> > What the test actually intended to do was to remap a virtual address
> > using MAP_FIXED. However, this operation unmaps the existing mapping and
> > creates a new one so the va is backed by a new page and MDWE is again
> > out of the picture, all remappings should succeed.
> >
> > This patch keeps the test case to make it clear that this situation is
> > expected to work.
> >
> > Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> > Fixes: 4cf1fe34fd18 ("kselftest: vm: add tests for memory-deny-write-execute")
> > ---
> >  tools/testing/selftests/mm/mdwe_test.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/testing/selftests/mm/mdwe_test.c b/tools/testing/selftests/mm/mdwe_test.c
> > index d0954c657feb..91aa9c3099e7 100644
> > --- a/tools/testing/selftests/mm/mdwe_test.c
> > +++ b/tools/testing/selftests/mm/mdwe_test.c
> > @@ -168,13 +168,10 @@ TEST_F(mdwe, mmap_FIXED)
> >       self->p = mmap(NULL, self->size, PROT_READ, self->flags, 0, 0);
> >       ASSERT_NE(self->p, MAP_FAILED);
> >
> > -     p = mmap(self->p + self->size, self->size, PROT_READ | PROT_EXEC,
> > +     /* MAP_FIXED unmaps the existing page before mapping which is allowed */
> > +     p = mmap(self->p, self->size, PROT_READ | PROT_EXEC,
> >                self->flags | MAP_FIXED, 0, 0);
> > -     if (variant->enabled) {
> > -             EXPECT_EQ(p, MAP_FAILED);
> > -     } else {
> > -             EXPECT_EQ(p, self->p);
> > -     }
> > +     EXPECT_EQ(p, self->p);
> >  }
>
> This is just validating the MDWE doesn't block a MAP_FIXED replacement?

Yes! :)

In v4 I modified the description just a little bit to state that out clearly.